The invention relates to power transistor structures in which a large number of individual elements are coupled in parallel to act as a single unit.
In linear applications, maximum transistor stress occurs at higher voltages when considerable power is being dissipated for longer than 1 ms. Because the emitter base voltage of bipolar transistors has a negative temperature coefficient, the tendency is for maximum conduction where temperature is highest. The resulting hot spot formation can become unstable, leading to secondary breakdown. This can be avoided by ensuring uniform conduction.
The problem of localization can be better understood by considering two parallel connected transistors. The following equations account for the transconductance and thermal effects. ##EQU1## Where: g.sub.m =base to collector transconductance
q=electron charge PA1 k=Boltzmann's constant PA1 T=Absolute temperature PA1 .theta..sub.j =Thermal resistance of the transistor PA1 .phi..sub.B =Temperature coefficient of the transistor base to emitter voltage. PA1 I.sub.C =Collector Current PA1 V.sub.CE =Collector to emitter voltage PA1 V.sub.BE =Base to emitter voltage. PA1 V.sub.bal is the voltage across the ballast resistor PA1 V.sub.B is the combined V.sub.bal and V.sub.BE.
Current sharing becomes potentially unstable when V.sub.BE decreases with increasing I.sub.C. Without ballasting, this can occur for a temperature rise greater then 10.degree. C. These equations are only valid when the g.sub.m of the parallel connected transistors is equal. Once an instability is initiated, the mathematical description is less concise. The process exhibits hysteresis, and the destabilization will persist until .DELTA.T is reduced to some lower value.
The situation can be improved with resistive ballasting in which a resistor is inserted in series with each emitter element. (A base ballast resistor can be moved to the emitter for analysis by dividing its resistance by transistor current gain.) The following related equations show that sufficient conditions for stability are simple and straightforward. ##EQU2## Where: R.sub.bal is the ballast resistance
A fraction of volt drop across the ballast resistor will insure uniform conduction for .DELTA.T=300.degree. C., even if low level injection is assumed (.phi..sub.B =2.5 mV/.degree.C.) and mutual thermal coupling between elements is ignored. This shows the forward biased secondary breakdown can be completely eliminated for any reasonable set of operating conditions, suffering only an insignificant increase in transistor losses.
Standard bipolar power transistors rely upon high level effects rather than ballast to reduce g.sub.m and .phi..sub.B. The high level threshold can be lowered by using wide emitters and thick, lightly doped bases; but the improvement available is rather limited. Such structures are inherently slow. Single-diffused transistors also have the collector-base junction located close to the heat sink, which reduces thermal resistance and improves thermal coupling among elements.